1. Field of the Invention
The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing with liquid flow.
2. Related Art
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in photolithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.
U.S. Pat. Nos. 4,953,388 and 4,550,592 disclose an alternative approach to proximity sensing that uses an air gauge sensor. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers.
Furthermore, as imaging requirements within lithography methods become more challenging, one alternative approach being used is immersion lithography. Within immersion lithography the gap between the last lens in the projection optic box and a wafer is filled with a liquid to enhance system performance. Such an approach supports printing of smaller feature sizes. In these systems, a wafer to be worked is surrounded by a pool of the liquid. Air gauge sensors, such as those disclosed in U.S. Pat. Nos. 4,953,388 and 4,550,592, would be ineffective in an immersion lithography system.
Immersion lithography systems are generating considerable interest within the microlithography community. The technology enables the index of refraction in the image space, and thus the numerical aperture of the projection system to be greater than unity. As a result, the technology has the potential to extend 193 nm tools used in lithography down to 45 nm, and possibly below. For an immersion lithography system to be effective, however, the index of refraction of the liquid surrounding the work surface must remain constant. Such variables as bubbling and temperature changes in the liquid can effect the index of refraction. A proximity sensor, therefore, must not induce bubbling or temperature changes, and ideally can reduce these effects.
What is needed is a liquid flow proximity sensor that can precisely sense distances in an immersion lithography system.